Re: [PATCH] pipe: bound new-page copies by remaining iterator bytes

[Mateusz Guzik]

On Thu, Oct 15, 2026 at 04:53:26PM +0800, Li Qiang wrote:
> anon_pipe_write() requests PAGE_SIZE bytes even when fewer bytes
> remain in the source iterator. This causes copy_page_from_iter()
> to perform an unnecessary extra iteration after copying all data.
> 
> Limit the copy size to the remaining iterator bytes to avoid
> this overhead without changing copy semantics.
> 
> UnixBench pipe results on Hygon C86 7490 (Hygon 4):
> 
> | Threads | Before | After | Improvement |
> |---------|--------|-------|-------------|
> | 1       | 835    | 854   | +2.28%      |
> | 256     | 179293 | 182895 | +2.01%     |
> 

If you are looking to improve speed here, the real gains would come from
redoing the loop with scoped_user_write_access_size machinery to avoid
repeat SMAP trips.

But that may be quite a bit of work. The patch as is seems to help so
I'm certainly not protesting it.

> Signed-off-by: Li Qiang <liqiang01@xxxxxxxxxx>
> ---
>  fs/pipe.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 292425a834dd..da5583f7f37d 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -595,7 +595,9 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
>  				break;
>  			}
>  
> -			copied = copy_page_from_iter(page, 0, PAGE_SIZE, from);
> +			copied = copy_page_from_iter(page, 0,
> +						     min_t(size_t, PAGE_SIZE,
> +							   iov_iter_count(from)), from);
>  			if (unlikely(copied < PAGE_SIZE && iov_iter_count(from))) {
>  				anon_pipe_put_page(pipe, page);
>  				if (!ret)
> 
> base-commit: 47324d3a5b3abd781295044d01d92d09f184e872
> -- 
> 2.43.0
>